Poison ivy is a problem commonly encountered while caching. In most people, it causes a rash, however some people are little or not sensitive to the irritant oil, which is called urushiol.
The plant is characterized by 3 notched leaves. The middle leaf looks like when you put your hands on top of one another, with the thumbs outward, and the outer leaves look like a mitten with a thumb. The plant is sometimes confused with Virginia Creeper, which looks similar to the poison ivy plant, only with 5 leaves. Both plants have hairy vines.
Some plants look similar (wild blackberry, for one) but are not poison ivy. As a rule of thumb: Leaves of three, leave it be.
The oil can remain active for years, it is NOT inactivated during winter. Specimens of urushiol several centuries old have found to cause dermatitis in sensitive people.
Some people may be immune to the urushiol, but it is a sensitizer, so most people will eventually become allergic after repeated contact.